When a projection system projects an image onto a screen, the projected image can be out of focus, or partially out of focus, if the screen is not positioned perpendicular to the main optical axis of the projection system. This is a particular problem in systems using facetted or curved projection screens where the screen-forming elements can be angularly offset with respect to the optical axis of the projection system.
It is possible to correct this focus error without the need to reposition the screen and/or projector. The position of the projection lens can be adjusted with respect to the projector. This solution is generally known as the “Scheimpflug principle”. The action performed on the projection lens itself is called Scheimpflug adjustment or correction.
One approach to Scheimpflug adjustment in a projection system uses an adjustment system comprising a carrier plate connected to the projection lens and a base plate connected to the projector. The carrier plate can be rotated with respect to the base plate about a horizontal rotation axis positioned above the lens and the carrier plate can be rotated with respect to the base plate about a vertical axis positioned at the side of the lens. Adjustment bolts and compression springs connect the carrier plate and base plate. However, tilting the projection lens also defocuses it strongly. This requires refocusing of the lens by moving the lens carrier plate along the Z-axis by adjusting all three of the bolts by exactly the same amount. The focus (or back focal length) and tilt (or Scheimpflug) adjustments are combined, typically requiring iterative adjustment until a satisfactory combination of Scheimpflug correction and focus is achieved.
Another approach to Scheimpflug adjustment in a projection system uses ball-joint or spherically-shaped joints between a carrier plate connected to the projection lens and a fixed plate connected to the base plate or X-Y position adjustment mechanism. A spherically-shaped carrier plate can rotate within a spherically-shaped base plate. It can be difficult to make a smooth adjustment with this arrangement as it is a friction-based solution with undesirable stick-slip effects. Also, it is difficult to lock the position of the projection lens after adjustment. This solution is also expensive because of the smooth curved surfaces needed. Accordingly, this solution is not suitable for heavy projection lenses.
Another approach to Scheimpflug adjustment in a projection system is described in EP 1 566 677A1. A projection lens is mounted on a base plate. The position of the projection lens can be adjusted by applying horizontal and/or vertical shifts in a plane perpendicular to the optical axis of the projection lens. The projection lens can also be tilted to provide Scheimpflug adjustment.